3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), together referred to as High Speed Packet Access (HSPA), are also mobile communication protocols developed to cope with higher data rates than original UMTS protocols were capable of. The Universal Terrestrial Radio Access (UTRA) Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
FIG. 1a illustrates a radio access network with an RBS 101 that serves a UE 103 in a cell 105. In UMTS, also referred to as a 3G system, a Radio Network Controller (RNC) 106 controls the RBS 101 and other neighboring RBSs, and is, among other things, in charge of management of radio resources in cells for which the RNC is responsible. The RNC is in turn also connected to the core network. In GSM, also referred to as a 2G system, the node controlling the RBS 101 is called a Base Station Controller (BSC) 106. FIG. 1b illustrates a radio access network in an LTE system, also referred to as a 4G system. An eNB 101a serves a UE 103 in the cell 105a. The eNB 101a is directly connected to the core network. The eNB 101a is also connected via an X2 interface to a neighboring eNB 101b serving another cell 105b. 
Signal Measurements for Mobility
Signal measurements performed by a UE can be used for various purposes. In particular, these measurements may be used for mobility-related tasks such as cell selection and reselection and handover, but also for positioning, Self-Organized Network (SON) management, network planning, and Minimization of Drive Tests (MDT). Signal strength and signal quality are the general parameters used for signal measurements.
In UTRAN, the following three downlink neighbor cell measurements are specified primarily for mobility purposes:
Common Pilot Channel (CPICH) Received Signal Code Power (RSCP)
UTRA carrier Received Signal Strength Indicator (RSSI)
CPICH Ec/No, where CPICH Ec/No=CPICH RSCP/carrier RSSI
The RSCP is measured by the UE on a cell-level basis, using the CPICH. The UTRA carrier RSSI is measured over the entire carrier. It corresponds to the total received power and noise from all cells including serving cells on the same carrier. The above CPICH measurements are the main quantities used for mobility decisions.
In E-UTRAN the following two downlink neighbor cell measurements are specified, also primarily for mobility purposes:
Reference Symbol Received Power (RSRP)
Reference Symbol Received Quality (RSRQ), where RSRQ=RSRP/carrier RSSI
RSRP in E-UTRAN is solely measured by the UE on a cell-level basis, using reference symbols (RS). The E-UTRA carrier RSSI is measured over the configured measurement bandwidth up to the entire carrier bandwidth. Again, the RSSI is the total received power and noise from all cells, including serving cells, on the same carrier. These two RS based measurements are also the main quantities that are likely to be used for the mobility decisions.
In GSM the following measurement is specified:
GSM Broadcast Channel (BCCH) carrier RSSI
CDMA-2000 1×RTT is a 3G wireless technology based on Code Division Multiple Access (CDMA). CDMA-2000 1×RTT is a CDMA version of the IMT-2000 standard which was developed by the International Telecommunication Union (ITU). In cdma2000 1×RTT system the following quality measurement for mobility is specified:
CDMA2000 1×RTT Pilot Strength
In CDMA-2000 High Rate Packet Data (HRPD) system the following quality measurement for mobility is specified:
CDMA2000 HRDP Pilot Strength
IEEE 802.16 is a series of Wireless Broadband standards authored by the Institute of Electrical and Electronics Engineers (IEEE). In WiMAX (Worldwide Interoperability for Microwave Access) IEEE 802.16 systems the following measurements are used for mobility:
WiMAX Preamble Carrier to Interference and Noise Ratio (CINR)
WiMAX RSSI.
WiMAX Preamble CINR is the CINR of the WiMAX downlink preamble, measured by the UE for a particular base station. This measurement quantity provides information on the actual operating condition of the receiver, including interference and noise levels, and signal strength. It therefore depicts the cell quality and is analogous to RSRQ and CPICH Ec/No in E-UTRAN and UTRAN respectively.
WiMAX RSSI is the Received Signal Strength Indicator measured by the UE from the downlink preamble for a particular base station. It corresponds to signal strength measurements RSCP in UTRAN or RSRP in E-UTRAN.
Neighbor cell measurements are typically averaged over a long time period to filter out the effect of fast fading. The measurements may e.g., be averaged over a time period in the order of 200 milliseconds or even longer. There is also a requirement on the UE to measure and report neighbor cell measurements such as RSRP and RSRQ in E-UTRAN for a certain minimum number of cells. In both UTRAN and E-UTRAN the requirement is to measure eight cells, comprising one serving and seven neighbor cells, on the serving carrier frequency. Such a measurement is commonly termed an intra-frequency measurement.
Timing Measurements
UE timing measurements are e.g., used for fingerprinting positioning and Observed Time Difference Of Arrival (OTDOA) in LTE. However, such measurements may also be used for mobility purposes, network planning, SON, and MDT.
The following non-satellite based UE timing measurements are currently standardized and can be used at least for positioning purposes in LTE:
UE Rx-Tx time difference, currently defined only for intra-frequency measurements. The UE Rx-Tx time difference is defined as TUE-Rx−TUE-Tx, where TUE-Rx is the UE received timing of downlink radio frame number i from the serving cell, defined by the first detected path in time, and TUE-Tx is the UE transmit timing of uplink radio frame number i.
Reference Signal Time Difference (RSTD), defined for intra- and inter-frequency measurements. RSTD is the relative timing difference between the neighbor cell j and the reference cell i, defined as TSubframeRxj−TSubframeRxi, where TSubframeRxj is the time when the UE receives the start of one subframe from cell j, and TSubframeRxi is the time when the UE receives the corresponding start of one subframe from cell i that is closest in time to the subframe received from cell j. The reference point for the observed subframe time difference shall be the antenna connector of the UE.
The following non-satellite based timing measurements are currently standardized and may be used for positioning in UTRAN (3GPP TS 25.215, v10.0.0, 5.1.8-5.1.10, 5.2.8, 5.2.10, 5.2.14):
UE measurements (3GPP TS 25.215, v10.0.0, 5.1.8-5.1.10)
SFN-CFN observed time difference
SFN-SFN observed time difference
UE Rx-Tx time difference
UTRAN measurements (3GPP TS 25.215, v10.0.0, 5.2.8, 5.2.10, 5.2.14)
Round trip time
PRACH Propagation delay
SFN-SFN observed time difference
Mobility Scenarios
Fundamentally, there are two kinds of UE mobility states:
Low activity state mobility such as cell reselection;
Connected state mobility such as handover, cell change order, Radio Resource Control (RRC) re-direction upon connection release.
In LTE there is only one low activity mobility state called idle state. In HSPA there are the following low activity states:
Idle State
URA_PCH state (UTRAN Registration Area Paging Channel state)
CELL_PCH state (Cell Paging Channel state)
CELL_FACH state (Cell Forward Access Channel state)
In HSPA systems, the connected state is also called CELL_DCH state since at least one Dedicated Channel (DCH) is in operation, at least for the maintenance of the radio link quality.
In any low activity state, the UE autonomously performs cell reselection without any direct intervention of the network. However, to some extent the UE behavior in low activity mobility state scenario could still be controlled by a number of broadcasted system parameters and performance specifications. The handover on the other hand is fully controlled by the network through explicit UE specific commands and by performance specification. Similarly, a RRC re-direction upon connection release mechanism is used by the network to re-direct the UE to change to another cell which may belong to the Radio Access Technology (RAT) of the serving cell or to another RAT. In this case, the UE typically goes into idle state upon receiving the ‘RRC re-direction upon connection release’ command, searches for the indicated cell or RAT, and accesses the new cell or RAT.
In both low activity state and connected state, the mobility decisions are mainly based on the same types of downlink neighbor cell measurements that were discussed above.
Both UTRAN and E-UTRAN are frequency reuse-1 systems. This means that the geographically closest cells or adjacent neighbor cells operate on the same carrier frequency. An operator may also deploy multiple frequency layers or carriers within the same coverage area. Therefore, idle mode and connected mode mobility in both UTRAN and E-UTRAN could be broadly classified into three main categories:
Intra-frequency mobility for low activity and connected states
Inter-frequency mobility for low activity and connected states
Inter-RAT mobility for low activity and connected states
In intra-frequency mobility, the UE moves between cells belonging to the same carrier frequency. This is the most important mobility scenario since it involves low cost in terms of delay, as mobility measurements can be carried out in parallel with channel reception. In addition, an operator would have at least one carrier at its disposal that the operator would like to be efficiently utilized.
In inter-frequency mobility, the UE moves between cells belonging to different carrier frequencies but of the same RAT. This could be considered as the second most important scenario.
In inter-RAT mobility, the UE moves between cells that belong to different RATs such as between UMTS and GSM or vice versa, or between UMTS and LTE or vice versa.
Positioning Methods
The following positioning methods are available or are likely to be introduced in the HSPA and LTE standard for both the control plane and the user plane solution:
Fingerprinting or pattern matching;
Cell Identification (CID);
UE-assisted and network-based Enhanced CID (E-CID), including network-based angle of arrival (AoA);
UE-based and UE-assisted Assisted Global Navigation Satellite System (A-GNSS) including (Assisted Global Positioning System (A-GPS);
UE-assisted OTDOA.
Some of them are described below in more detail.
Fingerprinting or Pattern Matching:
The fingerprinting or pattern-matching-based positioning method is characterized by two main phases. During the first phase, which is the offline phase, the location fingerprints are created by performing a site-survey. The site or the coverage area is sub-divided into a rectangular grid of points. During the offline phase, one or more types of measurements such as received signal strength, signal quality, path loss, time difference of arrival, etc., from the serving and multiple neighboring cells are performed. That is, the UE measurements mentioned in the preceding sections can be used. Statistics of the obtained measurement are used to create a database or 2-dimensional table containing predetermined measurement values, which values are mapped to the points of the grid. Thus, the vector of the measurement values at a point on the grid is called the location fingerprint of that point. Measurements during the offline phase can either be obtained by using a mobile terminal or by a suitable dedicated device, which is capable of detecting cells and performing the required measurements from the detected cells. Thus, the objective of the offline training phase is to build the mobile user's location profile. During the second phase, or the so-called online phase, the mobile terminal whose position is to be determined performs measurements, such as received signal strength, from the serving and several neighbor cells. The positioning node then computes the user's location, i.e., the location of the mobile terminal, by determining the best match between the mobile reported measurements and those corresponding to the location fingerprints in the pre-defined database. The best matching location fingerprint is then reported to the mobile terminal as the estimated position.
E-CID Positioning:
E-CID positioning exploits the advantage of low-complexity and fast positioning with Cell Identification (CID), which exploits the network knowledge of geographical areas associated with cell identities, but enhances positioning further with more measurement types. With E-CID, the following sources of position information are involved: the CID and the corresponding geographical description of the serving cell, the Timing Advance (TA) of the serving cell, and the CIDs and the corresponding signal measurements of the cells (up to 32 cells in LTE, including the serving cell), as well as angle-of-arrival (AoA) measurements.
The following UE measurements can be utilized for E-CID in LTE: RSRP, RSRQ, and UE Rx-Tx time difference. The E-UTRAN measurements available for E-CID are eNodeB Rx-Tx time difference also called TA Type 2, TA Type 1 being (eNodeB Rx-Tx time difference)+(UE Rx-Tx time difference), and uplink (UL) AoA. UE Rx-Tx measurements are typically used for the serving cell, while e.g., RSRP and RSRQ as well AoA can be utilized for any cell and can also be conducted on a frequency different from that of the serving cell.
UE E-CID measurements are reported by the UE to a positioning server such as the Evolved SMLC (E-SMLC) or the Secure User Plane Location (SUPL) Location Platform (SLP) in LTE, over the LTE Positioning Protocol (LPP). The E-UTRAN E-CID measurements are reported by the eNodeB to the positioning node over the LPP Annex protocol (LPPa).
The UE may receive assistance data from the network. However, no LPP assistance for E-CID is currently specified in the standard.
OTDOA Positioning:
The OTDOA positioning method makes use of the measured timing of downlink signals received from multiple eNodeBs at the UE. The UE measures the timing of the received signals using assistance data received from the positioning node, and the resulting measurements are used to locate the UE in relation to the neighboring eNodeBs.
With OTDOA, a UE measures the timing differences for downlink reference signals received from multiple distinct locations. For each neighbor cell, the UE measures RSTD, which is the relative timing difference between neighbor cell and reference cell. The UE position estimate is then found as the intersection of hyperbolas corresponding to the measured RSTDs. At least three measurements from geographically dispersed base stations with a good geometry are needed to solve for two coordinates of the terminal and the receiver clock bias. In order to solve for position, precise knowledge of the transmitter locations and transmit timing offset is needed.
To enable positioning in LTE and to facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, new physical signals dedicated for positioning, so called Positioning Reference Signals (PRS) have been introduced and low-interference positioning subframes have been specified in 3GPP.
PRS are transmitted from one antenna port (R6) according to a pre-defined pattern. A frequency shift that is a function of Physical Cell Identity (PCI) can be applied to the specified PRS patterns to generate orthogonal patterns that model the effective frequency reuse of six, making it possible to significantly reduce neighbor cell interference on the measured PRS and thus improve positioning measurements. Even though PRS have been specifically designed for positioning measurements and in general are characterized by better signal quality than other reference signals, the standard does not mandate using PRS. Other reference signals, e.g., Cell-specific Reference Signals (CRS) could in principle also be used for positioning measurements.
PRS are transmitted in pre-defined positioning subframes grouped by several consecutive subframes, i.e., one positioning occasion. Positioning occasions occur periodically with a certain periodicity of N subframes, i.e., the time interval between two positioning occasions. The standardized periods N are 160, 320, 640, and 1280 ms, and the number of consecutive subframes may be 1, 2, 4, or 6.
Carrier Aggregation
To enhance peak-rates within a technology, multi-carrier or Carrier Aggregation (CA) solutions are known. For example, it is possible to use multiple 5 MHz carriers in HSPA to enhance the peak-rate within the HSPA network. Similarly, in LTE, multiple 20 MHz carriers may for example be aggregated in the UL and/or the downlink (DL). Each carrier in a multi-carrier or CA system is generally termed as a component carrier (CC), or sometimes also as a cell. In simple words, the CC means an individual carrier in a multi-carrier system. CA is sometimes called multi-cell operation, multi-carrier operation, or multi-carrier transmission and/or reception. This means that CA can be used for transmission of signals and data in UL and DL. In a CA deployment, one of the CCs is the primary CC/cell or anchor CC/cell, while the remaining ones are called secondary or supplementary CCs/cells. Generally, the primary or anchor CC/cell carries the essential UE-specific signaling. The primary CC/cell exists in both UL and DL. The network may assign different primary CCs/cells to different UEs operating in the same sector or cell.
The CCs/cells belonging to the CA system may belong to the same frequency band, so called intra-band CA, or to different frequency bands, so called inter-band CA, or any combination thereof such as two CCs/cells in band A and one CC/cell in band B. The inter-band CA comprising of CCs/cells distributed over two bands is also called dual-band-dual-carrier-HSDPA (DB-DC-HSDPA) in HSPA, or inter-band CA in LTE. Furthermore, the CCs/cells in intra-band CA may be adjacent or non-adjacent in the frequency domain, which is also known as intra-band non-adjacent CA. A hybrid CA comprising of intra-band adjacent, intra-band non-adjacent and inter-band is also possible. Using carrier aggregation between carriers of different technologies is also referred to as multi-RAT CA, or multi-RAT-multi-carrier system, or simply inter-RAT CA. For example, the carriers from UMTS and LTE may be aggregated. Another example is the aggregation of LTE and CDMA2000 carriers. For the sake of clarity, CA within a same technology as described above may be called intra-RAT or simply single RAT CA.
Problem Description
It is not mandatory for 3GPP Release 8 multi-mode UEs to support UTRA to E-UTRA measurements in CELL_DCH state. There is a feature group indicator that can indicate whether or not such a measurement is supported by the UE. Hence, the network may not have E-UTRAN measurements carried out by the UE as a basis for a decision on when to handover or redirect the UE to a E-UTRAN cell. Such a decision then becomes blind, and may result in the UE being forced to revert to UTRAN or GSM in the event that the E-UTRAN cell coverage is bad in the area where the UE is when it receives the handover or connection release with redirection command.
Furthermore, there is no support for UTRA to E-UTRA measurements for UEs in Cell_FACH state in 3GPP Releases 8 to 10. Hence, a UE in the CELL_FACH state camping on a UTRAN cell cannot reselect an E-UTRAN cell. At the same time, it has been observed that a UE may stay longer in the CELL_FACH state than what was initially assumed in the standardization. Therefore, the UE may get stuck in UTRA even though it may be in good coverage of a higher prioritized E-UTRAN carrier.
Circuit Switched Fallback (CSFB) is introduced in 3GPP Release 8 to allow an UE in LTE to reuse circuit switched domain services by defining how the UE can switch its radio from an E-UTRAN access to another RAT access such as GSM or UTRAN access that can support circuit switched domain services. In CSFB scenarios, the UE is connected to or camping on an E-UTRAN cell, and is redirected, e.g., to an UTRAN cell when the UE receives an incoming call. This UTRAN cell might not have been measured before by the UE, and in order to minimize the interruption time, the eNB may tunnel system information for the target cell in UTRAN. However, the UE still needs to detect the cell. In case of several neighbor UTRAN cells it may take the UE some time to find the target cell if it is not the strongest cell on that carrier as perceived by the UE.